1. Field of the Invention
The invention relates to a water resistant molding material (sometimes called as modeling material), and a producing method for the water resistant molding material. In particular, the invention relates to a water resistant molding material that can be stably manufactured while being excellent in water resistance after dried, and a producing method for the water resistant molding material.
2. Description of the Related Art
Light weight molding materials (which may be referred to as light clays hereinafter) that are ready for molding and transport have been widely used in recent years as clays used for handcrafts. Such light clays can be obtained by blending hollow microspheres as fillers with a polyvinyl alcohol resin, a fiber powder and water in a predetermined proportion.
However, when organic hollow microspheres or inorganic hollow microspheres are added to the polyvinyl alcohol resin, the latter is flocculated while viscosity, fluidity and cohesivity, etc. of the light clay decrease within a short period of time.
Accordingly, a light clay, that is not cracked, broken or fractured even by being severely deformed by an external force after molding and drying while long term storage is possible, has been proposed for solving above-mentioned problems (see, for example Japanese Patent Application Laid-Open (JP-A) No. 2001-131329; claims and examples).
More specifically, the light clay is composed of synthetic resin hollow microspheres (from 5 to 15% by weight) with a particle diameter of 20 to 120 μm as principal materials, a polyvinyl alcohol resin (from 5 to 10% by weight), a vinyl acetate resin and water (from 50 to 80% by weight). A bending ratio between the polyvinyl alcohol resin and the vinyl acetate resin is in the range from 10:7 to 10:3 by weight.
Another proposed light clay is also quite excellent in deformation resistance when dried with excellent properties such as workability and soft touch feeling as clays used for handcrafts (see, for example, JP-A-2001-234081; claims and examples).
More specifically, this light clay for handcrafts is composed of synthetic resin hollow microspheres (from 5 to 15% by weight) with a particle diameter of 20 to 120 μm, a polyvinyl alcohol resin (from 5 to 10% by weight), a vinyl acetate resin (from 1.5 to 7% by weight) containing a plasticizer, polyethylene oxide (from 0.5 to 1.5% by weight) and water (from 50 to 80% by weight).
However, no attention has been paid on the concentration of acetate ions in the light clays disclosed in JP-A-2001-131329 and JP-A-2001-234081, and the manufacturing characteristics have been unstable, while the clay obtained is poor in water resistance.
Since the vinyl acetate resin containing the plasticizer is an essential component of the light clay disclosed in JP-A-2001-131329, the polyvinyl alcohol resin is flocculated when stored for a long period of time under hermetically sealed conditions, and viscosity and plasticity of the light clay are remarkably deteriorated.
The content of a binder resin (such as a polyvinyl alcohol resin, vinyl acetate resin containing a plasticized, or polyethylene oxide) is too large in the light clays disclosed in JP-A-2001-131329 and JP-A-2001-234081 while the blending ratio between the binder resin and water is not optimized. For this reason, the molding material obtained has been poor in fluidity as well as in shape-keeping ability.